The present invention relates to an impregnated cathode capable of stably forming an atomic layer of low work function on the cathode surface.
Impregnated cathodes of high current density are promising for production of display tube, picture tube, pickup tube, etc of high resolution. As disclosed in Japanese Patent Unexamined Publication No. 61-183838, conventional impregnated cathodes comprise a cathode obtained by impregnating pore portions of a sintered W body with an electron emissive material and a thin film formed on the surface of the cathode, comprising a high melting metal and Sc or an Sc oxide, or of a high melting metal, Sc and a Sc oxide, or of a high melting metal and a compound of Sc, W and O. These cathodes are characterized by, during operation, having on the cathode surface a Ba/Sc/O complex layer of singe to several molecules and having a low work function.
In the above impregnated cathodes according to the prior art, no attention is paid to the point that the thin film formed on the cathode surface undergoes deterioration owing to thermal oxidation during the sealing off step of tube production. This deterioration of the thin film owing to thermal oxidation makes it impossible for the above impregnated cathodes to exhibit the intended properties.